Digital transmission networks, such as those based on Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standards, are used extensively for transporting broadband communications signals. In some applications, these broadband communications signals may include base rate signals, which are signals having a fundamental rate or structure. Base rate signals are typically combined with other similar base rate signals to create a higher rate and more complex signal. Using SONET as an example, a synchronous transport signal (STS-1) having a fundamental rate of 51.84 Mbps may be a base rate signal, and an STS-12 signal may comprise twelve (12) individual STS-1 signals to form a base rate signal group (BRSG) having a composite rate of 622.08 Mbps.
To achieve transmission efficiencies as well as other performance advantages, it is desirable in some applications to switch individual signals as a single, composite group rather than on an individual basis. For example, it may be advantageous to maintain equalized or uniform path delay for signals that are transported through a network. Moreover, it may also be desirable to perform group switching based on selected performance-related attributes, such as signal quality, which can be conveyed in the form of signal status for each of the signals. Building on the previous SONET example in a switching application, it may be desirable to switch the entire STS-12 base rate signal group as a composite, higher rate signal through the network rather than individually switching each of the lower rate STS-1 signals within that STS-12 signal group. In the prior art, complex group switches are typically used for this purpose.
Complex group switches are known to have a complex switching structure comprising multiple, cascaded selection/switching stages with highly coupled and complex control structures. In order for the complex group switch to appropriately switch individual signals as a composite group, the signal status for each of the individual signals must be resolved to derive a group status at each particular selection/switching stage. This resolution of signal status adds excessive processing overhead which results in undesirable switching delays during signal transmission.
Out-of-band control techniques, which are typically used in prior art group switches to resolve group signal status, can also add further complexity to a group switch. For example, each selection stage in a prior art group switch will typically require its own complex control structure to provide group status information. The control structures for each selection/switching stage must also be closely coupled to preserve the group status as signals are transported through the stages. Additionally, an overall control structure may also be required to couple the control structures from each individual selection/switching stage along the transmission path. It is also known that the complexity of control increases significantly for configurations having distributed switching elements spanning multiple selection/switching stages.